Structural panels such as aircraft fuselage panels, panels of automobiles, panels found on machinery, and panels found in household appliances, typically radiate noise due to vibratory motion induced in the panels. The resonant vibrations of the structural panels are often induced by unavoidable external sources. For example, engines, motors, compressors, etc., may induce vibrations in panels. Noise problems with structural panels are more apparent when panel thickness is reduced to minimize panel weight, such as in aircraft fuselage panels and other aerospace applications.
One known technique frequently employed to reduce resonant vibrations in structural panels is the use of viscoelastic damping treatments. In free-layer type damping treatments, a viscoelastic damping material, such as rubber, is added as a free layer to the surface of the structural panel. The damping treatment is usually applied to the entire surface of the panel. The viscoelastic material absorbs a portion of the total vibration energy by shear deformation. A more effective damping technique is to cover the free layer of viscoelastic material with a constraining layer of metal to form a constrained-layer type damping treatment. The addition of the constraining layer on top of the free layer improves the energy absorption characteristics of the damping layer.
Although the conventional damping treatments provide increased damping for resonant modes of the structural panel, the large amounts of viscoelastic material which are used to cover the entire surface of the structural panel are expensive and heavy. These conventional damping treatments are particularly disadvantageous for aerospace applications or any other applications in which thin, light panels are desired.